The invention relates to a mixing apparatus for producing a liquid suspension of solids with uniform concentration.
Many chemical processes involve mixing operations that suspend solids in liquids such as mixing, crystallization, reaction and slurry suspension. Handling floating solids is often a significant aspect of a mixing operation. Solids may float for any number of reasons including low solid density, low bulk density and/or the non-wetting characteristics of a particular solid. Such solids float on a liquid""s surface exposed to air and experience effects such as air entrainment. The ability to make a slurry of such materials typically requires the steps of submergence, degassing and distribution. Prior art solutions for distributing floating solids assumed that multiple levels of impellers are required. Most prior art devices positioned an impeller near the liquid/air surface to engage the floating solids and located at least one other impeller beneath the surface of the liquid in order to continue mixing the solids as they became submerged. However, for many operations such agitation results in the application of high levels of shear which cause product damage, especially if the solids are friable particulates. This is especially true if the mixing apparatus is being used in a transfer operation where batch operations may be subject to several hours of holdup time while experiencing extended periods of agitation. Prior art designs with multiple impellers for combining solids and liquid are described in U.S. Pat. No. 5,399,014 (Takata et al.), U.S. Pat. No. 4,614,439 (Brunt et al.), U.S. Pat. No. 4,934,828 (Janssen) and U.S. Pat. No. 4,552,463 (Hodson). Mixing that involves high energy input into a slurry may promote uniformity but is undesirable if product damage results.
Moreover, if the mixing tank is used as a feed tank to another operation in a chemical process, such as a drying operation, there is a need to avoid solids from settling in the bottom of the tank. The solids tend to remain in the tank at the end of a batch and accumulate from batch to batch. This is an unstable situation that may produce batch-to-batch non-uniformity and must eventually be addressed by shutting down the process and cleaning the tank when the solids concentration of the slurry becomes too high over time. Agitated vessels that can either maintain a uniform suspension or resuspend settling solids are desirable.
There remains a need for an agitated vessel design that produces a uniform concentration of solids in a liquid within a tank as well as in the effluent stream leaving the tank. The system should deliver a uniform concentration regardless of whether the solids tend to float or sink. The uniform concentration should be maintained from a full level in the mixing tank to as low of a level in the tank as is possible. The design should provide a maximum working volume in the tank and leave a minimum accumulation of slurry at the bottom of the tank as the tank is emptied. Finally, the system should create a low shear environment in order to avoid product damage.
The invention provides an agitated vessel for producing a suspension of solids in liquid with uniform concentration. The vessel comprises a vertical cylindrical tank having sidewalls and a bottom with inclined surfaces forming an interior for holding a suspension of solids in liquid; means for feeding solids and liquid to the tank; a plurality of stationary baffles disposed in the interior of the tank and vertically extending the length of the tank; a rotating shaft vertically installed at the center of the interior of the vertical cylindrical tank; a single turbine impeller with vertical blades radiating from the rotating shaft, the lower edges of the blades being contoured to match the slope of the inclined surfaces of the bottom and positioned so that the lower edge of the impeller is close to the bottom; and an exit port.
Preferably, the impeller in conjunction with the baffles produces an overall bottom-to-top flow of the suspension along the sidewalls in the tank.
In a preferred embodiment, the exit port is located on the sidewalls approximately at the height of the impeller for withdrawing suspension having uniform concentration.